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This study aimed to conduct an inventory of fish assemblages in small streams that are tributaries of large reservoirs in three previously unstudied regions of the Upper Paraná River basin. We sampled 117 streams from 1^st to 3^rd order in Araguari (Nova Ponte hydropower plant reservoir), Paranaíba (São Simão reservoir) and Grande (Volta Grande reservoir) drainages. In total, 20,696 specimens belonging to 100 species, 53 genera, 20 families and six orders were collected. Of these, 11,530 specimens and 41 species were recorded in Araguari, whereas 17 were exclusive to this drainage. In Grande drainage, 3,537 individuals belonging to 41 species (11 exclusive) and in Paranaíba, 5,629 specimens and 67 species (38 exclusive) were sampled. The mean richness per stream was 6.7 for Grande, 9.0 for Araguari and 10.9 for Paranaíba drainage. The predominant orders were Characiformes, 48% of the total richness, and Siluriformes, 36%. Three species were diagnosed as new: Astyanax sp. 1 (Grande and Paranaíba), Astyanax sp. 2 and Astyanax sp. 3 (both from Araguari), all from scabripinnis group. Another three were considered potentially new: Characidium sp. 1, Characidium sp. 2 and Rhamdiopsis sp. Among all records, eight species are not native from the Paraná basin: the exotics Oreochromis niloticus and Tilapia rendalli and the allochtonous Cyphocharax gillii, Hoplerythrinus unitaeniatus, Knodus moenkhausii, Poecilia reticulata, Roeboides descalvadensis and Trichomycterus brasiliensis. This study fills part of the existing knowledge gap about fish from small streams of the Paraná basin. Inventories of the fish fauna from these low …